Midway through the xx's set at April's Coachella Valley Music and Arts Annual Festival in Indio, Calif., the London-based trio threw a cover song into its set: Aaliyah's smoldering 1997 single "Hot Like Fire." Solange, the R&B singer and friend of the xx, joined the band on stage to solidify one of the weekend's standout moments.

Guitarist and vocalist Romy Madley Croft, 23, talks modestly about her acclaimed band, but doesn't hesitate to gush about the two R&B singers who helped make the highlight possible.

"Aaliyah has been an artist I've grown up with, like an older sister," the soft-spoken Madley Croft said. "We were constantly playing her music around the house when I was growing up. It was apart of my life. ... When it came to Solange, we just think she's an incredible person and musician."

That combination of seductive R&B and the xx's dour, longing brand of indie-pop might seem strange to some, but spend time with the band's excellent albums (2009's "xx" and last September's "Coexist"), and the marriage makes sense. At times, the xx's music is difficult to categorize, but the sexy, slow-burn of Solange's recent work and Aaliyah's understated melodies fit the band's sound.

The music-obsessed trio — which also includes bassist/vocalist Oliver Sim and producer Jamie Smith, all best friends since age 11 — is influenced by a wide range of sounds, including '90s R&B, obscure house and calypso to name a few. But given the members' love of dance music, the xx's albums are surprisingly stark and minimalist. The result is a group that sounds like no other working band today.

But don't be fooled by the "minimalist" label. When the xx takes the Merriweather Post Pavilion stage on Sunday night, its deceptively powerful set will justify the large amphitheater setting.

Early on in the xx's young career, that was not the case. The band that quietly wrote its first album late at night in their bedrooms — in order to not wake up their parents — was not ready for large festival stages, Madley Croft admits. But when your group's debut record wins the Mercury Prize, an award given to the best British album of the year, there's little time to drag your feet.

"We've been thrown on so many different stages all over the world," she said. "We just had to embrace it."

Two years of constant touring in support of "xx" fortified the group's aplomb and stage presence, Madley Croft says. The band brought that confidence into a London studio and recorded "Coexist" between November 2011 and May 2012. When the album was completed, the band, whose members most often listen to music on headphones, "sat there and turned it up on the big speakers," Madley Croft says. As a result, the songs revealed themselves in ways that could reach even the most remote areas of large venues.

"This whole other, different part of the music comes out," she said. "The sub-bass, the stuff you can move to, as well as the subtleties. You get more of the rhythmic side. That's what we embrace."

The juxtaposition of the hushed vocals exchanged by Madley Croft and Sim with Smith's meticulously crafted, tranquil backdrops has made the xx one of the most exciting bands to emerge from London in years. Given the group's sparse, intimate arrangements, its also been labeled one of the quietest bands ever.

Madley Croft says that distinction is only half-right.

"We're not extroverts. We're not introverts," she said. "We're quite normal, not in-your-face people. But anyone that comes to see us live will see we're not quiet live. I love that people can feel the bass in their body."

Then, perhaps feeling a bit brash about the last comment, the polite Madley Croft quickly brings the xx back down to the ground.

"We don't ever want to be in anyone's face," she said. "We're not the kind of band to cause a scene."